In a WLAN (Wireless Local Area Network), to improve data transmission efficiency, a 4× symbol length is introduced into the next generation WLAN standard 802.11ax, and a symbol of 802.11a/n/ac is correspondingly referred to as a 1× symbol.
The 4× symbol length means that, in an OFDM (Orthogonal Frequency Division Multiplexing) symbol, a data length is 12.8 μs. Correspondingly, a percentage of a 3.2 μs CP in an OFDM symbol is (3.2/(3.2+12.8))=20%. This effectively improves transmission efficiency. It can be learnt that a time-domain transmission time of a data part changes from 3.2 us to 12.8 μs, which is a fourfold increase. In a frequency domain, there is a fourfold decrease in a bandwidth of each subcarrier correspondingly, because a smaller bandwidth indicates a longer transmission time. Specifically, for 802.11ac, there are 64 subcarriers at 20 MHz, corresponding to 64-point FFT, 128 subcarriers at 40 MHz, corresponding to 128-point FFT, and 256 subcarriers at 80 MHz, corresponding to 256-point FFT. For 802.11ax, there are 256 subcarriers at 20 MHz, corresponding to 256-point FFT, 512 subcarriers at 40 MHz, corresponding to 512-point FFT, and 1024 subcarriers at 80 MHz, corresponding to 1024-point FFT.
Using 20 MHz as an example, the 64 subcarriers of 802.11ac include 52 data subcarriers and four pilot subcarriers, and the 256 subcarriers of 802.11ax include 234 data subcarriers and eight pilot subcarriers. If a same MCS (Modulation and Coding Scheme) is used, a data volume that can be transmitted in 802.11ax is more than four times greater than a data volume that can be transmitted in 802.11ac, because 234>4*52. Results are the same for 40 MHz and 80 MHz.
After the 4× data symbol length is introduced, for a receive end, a time required for processing each OFDM symbol increases. A processing time at the receive end is mainly spent on: 1. FFT (Fast Fourier Transform); 2. demapping; 3. channel decoding. Channel decoding is the most time-consuming of the three. Because a data volume in each OFDM symbol increases, a channel decoding time increases. This processing delay becomes very serious in the case of a high bandwidth (such as 80 MHz) and/or a high MCS (such as MCS9).
After receiving some data frames or control frames that require immediate responses (respond after SIFS=16 μs), the receive end needs to first complete processing of the data frames or the control frames and then switch from a receiving state to a sending state. The processing and switching need to be completed within an SIFS (Short Interframe Space) time. For a 1× symbol length (that is, a frame of 802.11a/n/ac), the SIFS time of 16 us is sufficient for the receive end to complete data processing and status switching. However, for the 4× symbol length (that is, a frame of 802.11ax), data processing may result in a relatively long delay. As a result, within the current SIFS time of 16 μs, the receive end cannot complete data processing and status switching.